Group 2 trends in reactivity and solubility

Steps:

• Reactivity of group 2 metals increases down the group, so barium reacts faster than magnesium.
• Both magnesium and barium carbonates react with HCl to produce 1 mole of CO2 per mole of carbonate.
• Solubility of group 2 sulfates decreases down the group, making MgSO4 more soluble than BaSO4.
• Thermal stability of group 2 carbonates increases down the group, so MgCO3 decomposes more readily than BaCO3.

Why C is correct:

• Group 2 sulfate solubility decreases down the group due to increasing lattice energy relative to hydration energy, so MgSO4 is highly soluble while BaSO4 is insoluble.

Why the others are wrong:

• A: Barium's higher reactivity causes it to react more rapidly with HCl than magnesium.
• B: Both carbonates yield 1 mole of CO2 gas per mole with excess HCl, so equal amounts.
• D: Magnesium carbonate has lower thermal stability and decomposes more readily than barium carbonate.

Final answer: C